Air augmented rockets offer potential performance advantages over conventional rocket systems. The advantages inherent in air augmented rockets have long been recognized. The advantages result in using atmospheric air as an oxidizer, supplanting a significant fraction of the oxidizer that must be transported in a more conventional rocket. One member of the air augmented propulsion family of rocket motors is the ducted rocket comprising a solid propellant fuel generator, a booster propellant, a secondary combustion chamber including air inlets, and exhaust nozzle. The secondary combustion chamber typically houses an integral solid propellant booster grain and ejectable nozzle. The ducted rocket has potential of delivering an effective specific impulse in the sustain phase (air augmented combustion phase) 50-60% greater than that of a conventional solid propellant powered rocket. This greater effective specific impulse leads to greater mission capability, e.g., longer range, higher velocity, greater payload, etc.
The purpose of a solid propellant fuel generator for a ducted rocket is to deliver to the secondary combustion chamber a combustible-rich exhaust capable of high heat release when combined with the ram air stream provided by the ducted rocket system. Light metals such as boron, aluminum and magnesium are attractive from a thermochemical performance standpoint as a fuel for a solid propellant fuel generator for use in a ducted rocket at high percentage metal loadings, greater than 50% by weight based on the weight of the propellant. One difficulty with light metal loaded propellants for fuel generators for ducted rockets has been low burning rates.
Castable magnesium-rich propellants based on hydrocarbon binders are difficult to formulate with burning rates much in excess of 0.9 in./sec. at 600 psia because of char formation which takes place during burning. The char formed partially inhibits combustion of the magnesium and impedes the progress of the flame front during burning. Higher burning rates, i.e., greater than 0.9 in./sec. are possible with magnesium-rich propellants based on hydrocarbon binders when ferrocenes or carboranes are employed in the propellant compositions. However, these burning rate adjuvants are expensive and impart adverse aging and migration characteristics in propellants containing such materials.